1. Technical Field
The invention relates to an image processing apparatus for converting multilevel image information into binary image information when an image is formed.
2. Description of the Related Art
Generally, an image forming apparatus, such as a copying machine or a printer, using an electrophotographic system or an ink-jet technique, performs image processing for representing halftones by converting input multilevel image data into binary image data. As an image processing method for converting multilevel image data into binary image data, there is known a binarizing process (halftone-dot processing) for forming halftone dots (colored dots) having size corresponding to the input multilevel image data so that the density of a halftone image is reproduced apparently by the size of the colored dots.
For example, a color image forming apparatus for forming a color image by an electrophotographic system, prints and superposes respective toner images of the four colors of yellow (Y), magenta (M), cyan (C) and black (K) successively on a sheet of paper, which serves as a recording medium, to thereby form a color image. On this occasion, the density of each color toner image is reproduced as a set of a large number of fine halftone dots by use of the aforementioned binarizing process.
The binarizing process according to the related art will be described specifically while a color copying machine using an electrophotographic system is taken as an example. FIG. 13A is a view schematically showing an example of the binarizing process according to the related art. FIG. 13B is a view showing an example of halftone dots (halftone-dot image) generated by the binarizing process. As shown in FIG. 13A, to form an image of a color original, the image density in the color original is first detected in units of pixels each having a predetermined size, so as to generate multilevel data having a predetermined number of bits for representing the image density (density level “8” in 16 gradations of 4 bits is shown as an example in FIG. 13A) as an input image signal. Then, a comparator compares the input image signal (multilevel data) with predetermined screen pattern data (respective threshold data in a threshold matrix). Thus, multilevel data of each pixel in the color original is binarized to thereby generate a binarized recording signal.
The generated binarized recording signal is output to an exposure device. The exposure device controls the on/off state of an exposure beam (such as a laser beam) on the basis of the binarized recording signal so as to scan and exposes an image carrier (such as a photoconductor drum) with and to the exposure beam. Thus, an electrostatic latent image in accordance with the on/off state of the exposure beam is formed on the image carrier. The electrostatic latent image is developed with each color toner. In this manner, respective color toner images are formed. The respective color toner images are transferred and fixed onto a sheet of paper, so that a color image is formed. On this occasion, the image density of the color image on the sheet of paper is represented by halftone dots each having a size corresponding to the image density as shown in FIG. 13B.